Reading from a file in assembly

Question

I'm trying to learn assembly -- x86 in a Linux environment. The most useful tutorial I can find is Writing A Useful Program With NASM. The task I'm setting myself is simple: read a file and write it to stdout.

This is what I have:

section  .text              ; declaring our .text segment
  global  _start            ; telling where program execution should start

_start:                     ; this is where code starts getting exec'ed

  ; get the filename in ebx
    pop   ebx               ; argc
    pop   ebx               ; argv[0]
    pop   ebx               ; the first real arg, a filename

  ; open the file
    mov   eax,  5           ; open(
    mov   ecx,  0           ;   read-only mode
    int   80h               ; );

  ; read the file
    mov     eax,  3         ; read(
    mov     ebx,  eax       ;   file_descriptor,
    mov     ecx,  buf       ;   *buf,
    mov     edx,  bufsize   ;   *bufsize
    int     80h             ; );

  ; write to STDOUT
    mov     eax,  4         ; write(
    mov     ebx,  1         ;   STDOUT,
  ; mov     ecx,  buf       ;   *buf
    int     80h             ; );

  ; exit
    mov   eax,  1           ; exit(
    mov   ebx,  0           ;   0
    int   80h               ; );

A crucial problem here is that the tutorial never mentions how to create a buffer, the bufsize variable, or indeed variables at all.

How do I do this?

(An aside: after at least an hour of searching, I'm vaguely appalled at the low quality of resources for learning assembly. How on earth does any computer run when the only documentation is the hearsay traded on the 'net?)

Solution 1

you must declare your buffer in bss section and the bufsize in data

section .data
   bufsize dw      1024

section .bss
   buf     resb    1024

Solution 2

Ohh, this is going to be fun.

Assembly language doesn't have variables. Those are a higher-level language construct. In assembly language, if you want variables, you make them yourself. Uphill. Both ways. In the snow.

If you want a buffer, you're going to have to either use some region of your stack as the buffer (after calling the appropriate stack-frame-setup instructions), or use some region on the heap. If your heap is too small, you'll have to make a SYSCALL instruction (another INT 80h) to beg the operating system for more (via sbrk).

Another alternative is to learn about the ELF format and create a global variable in the appropriate section (I think it's .data).

The end result of any of these methods is a memory location you can use. But your only real "variables" like you're used to from the now-wonderful-seeming world of C are your registers. And there aren't very many of them.

The assembler might help you out with useful macros. Read the assembler documentation; I don't remember them off the top of my head.

Life is tough down there at the ASM level.

Solution 3

After the call to open, the file handle is in eax. You rightfully move eax it to ebx, where the call to read will look for it. Unfortunately, at this point you have already overwritten it with 3, the syscall for reading.

Comments

[jameshfisher]

I understand that buf is just a pointer to memory that I need to create, and that I need to request bufsize memory from the operating system. However, I don't how to do either of those things, and I can't find out.

[Borealid]

eegg : malloc is not actually a system call. You need to set up a heap. This is done, as I mentioned, with the brk and sbrk system calls. See man 2 brk. You need to look up the system call number corresponding to brk (see /usr/include/sys/syscall.h), then do a mov eax, # and int 80h to call it as per your above syscall pattern. Once you've done this, you have a heap ending at the address you specify! Neato!

[jameshfisher]

Okay, apparently brk is a system call, but sbrk isn't. sbrk looks decidedly more useful. Using just brk would be possible, if I knew where in memory the current program break is. Despite searching, I don't know how to find that. Looking for the source for sbrk (which presumably wraps brk), I can only find this, which I can make neither head nor tail of.

[Borealid]

@eegg What sbrk does is call brk(0) and record its return value (which is the current program break). Does that help?

[Lance]

What if you don't know the size of the file, how do you do that?

[Peter Cordes]

Why mess around with brk when you could just mmap(MAP_ANONYMOUS) to get however many pages you want? Or honestly for a toy program in need of a large buffer, a static buffer in the BSS is definitely the easiest choice. section .bss / mybuf: resb 1024*1024*1024 reserves 1GiB. (And no you don't want it in .data, that would actually put zeros in the executable file.) I'd love to +1 this answer for the good part explaining labels vs. high-level-language variables, though. But static variables do sort of map to label + storage; it's automatic storage that maps to regs.

[Peter Cordes]

The size is an assemble-time constant, you don't need to store it in memory. (And in a word makes little sense in 32-bit code). Use buf: resb 1024 / bufsize equ $-buf to have the assembler calculate a size for you that you can use as an immediate instead of having to load from memory.

[Nikola Petrovic]

If you declared a buffer of 1024 bytes, shouldn't the buffer size be given by db 1024, rather than dw 1024?